1. Technical Field
The present application relates generally to a handoff process in a cellular communication system and, in particular, to an apparatus and method for controlling a soft handoff between base stations using different operating frequencies in a CDMA (Code Division Multiple Access) terminal.
2. Description of the Related Art
In general, during a conversation on a mobile telephone, a handoff operation can occur between a source base station and a target base station (xe2x80x9cinter-station handoffxe2x80x9d) and within a given base station (xe2x80x9cintra-station handoffxe2x80x9d). The inter-station handoff operation is very important for maintaining call continuity during mobile communication. The conventional AMPS (Advanced Mobile Phone Service) system does not support a soft handoff method, which sometimes results in a call disconnection when a portable terminal (e.g., cellular phone) travels through the boundary between adjacent base stations. Similarly, a CDMA system does not support a soft handoff between base stations using the different frequencies, which results in call disconnection. The call disconnection or reception clatter which occurs during a conversation as a result of a conventional handoff operation is offensive to the user. Further, when the CDMA terminal provides a data communication service, a call disconnection during the data transmission may lead to loss of data being transmitted. Accordingly, the user cannot transmit data using the CDMA portable terminal.
Referring now to FIG. 1, a diagram illustrates a general handoff condition. In FIG. 1, reference characters A, B and C denote adjacent base stations having operating frequencies f1, f1 and f2, respectively. A portable terminal PT communicates with the base station to which it belongs by using the operating frequency allocated to the base station. When the portable terminal PT travels into coverage of an adjacent base station, however, the PT must be tuned to the frequency of the new base station.
Referring now to FIG. 2, a diagram illustrates a conventional portable CDMA terminal. A first local oscillator (LO) 13 generates a first local oscillation signal and a first mixer 12 mixes an RF (Radio Frequency) signal (e.g., the operating frequency signals from the base stations) received through an antenna 11 with the first local oscillation signal. A second LO 16 generates a second local oscillation signal and a second mixer 15 mixes the signal output from the first mixer 12 with the second local oscillation signal. The CDMA terminal also includes a RAKE receiver 30 which has a plurality of paths 18-21. Each of the paths 18-21 receives a signal which is output from the second mixer 15. A combiner 23 in the RAKE receiver 30 receives the signals from each of the paths 18-21 and independently outputs each of the signals to a decoder 24 which decodes the signal output from the combiner 23. A controller 40 generates signals for controlling the oscillation frequencies generated by the first and second LOs 13 and 16, for controlling the connection between the paths 18-21 and the combiner 23, and for controlling the operation of the decoder 24. The paths 18-21 are composed of fingers and the signal output from the second mixer 15 is applied to the fingers of the respective paths 18-21 at a specified time difference.
The conventional handoff operation will now be explained with reference to FIGS. 1 and 2. For instance, when base stations A and B, which use the same frequency f1, have the same frame offset, a soft handoff process is performed. On the other hand, a handoff between base stations A and C or between base stations B and C requires a frequency switching process since each pair of base stations utilizes different frequencies f1 and f2. The frequency switching operation can result in call disconnection. An area represented by AC in FIG. 1 denotes a common coverage area of base stations A and C. Since base stations A and C are shown using different frequencies (i.e., f1 and f2, respectively), the common area AC has both frequencies f1 and f2. The conventional portable terminal (FIG. 2), however, cannot simultaneously demodulate both operating frequencies f1 and f2 when it is located in common area AC since the portable terminal includes the single LO 13. Therefore, when the conventional portable terminal demodulates the RF signal received from the base station to a baseband signal, it can only process the signal received from the base station having the corresponding frequency. Therefore, as the portable terminal travels into the coverage area of base station B, the controller 40 must adjust the oscillating frequency of the LO 13 in order to demodulate the operating frequency of base station B. This frequency switching operation can lead to call disconnection.
The present application is directed to an apparatus and method for providing a soft handoff between base stations having different operating frequencies.
In one aspect, an apparatus for controlling soft handoff between a source base station generating a first operating frequency signal and a target base station generating a second operating frequency signal, comprises:
a first local oscillator for generating a first local oscillation signal;
a first mixer for mixing the first and second operating frequency signals with the first local oscillation signal;
a second local oscillator for generating a second local oscillation signal;
a second mixer for mixing a signal output from the first mixer with the second local oscillation signal;
a third local oscillator for generating a third local oscillation signal;
a third mixer for mixing the first and second operating frequency signals with the third local oscillation signal;
a fourth mixer for mixing an output signal of the third mixer with the second local oscillation signal;
a RAKE receiver having a plurality of main paths for receiving an output signal of the second mixer, an extra path for receiving an output signal of the fourth mixer, and a combiner for independently providing the signals provided to the main paths and the extra path to an output node thereof;
a decoder for decoding a signal output from the combiner; and
a controller for controlling the first and third local oscillators, the RAKE receiver and the decoding means, such that when the apparatus moves from an operating area covered by the source base station into an operating area covered by both the source base station and the target base station, the controller causes the first local oscillator to generate the first local oscillation signal to demodulate the first operating frequency signal, enables the third local oscillator to generate the third local oscillation signal to demodulate the second operating frequency, enables the extra path for receiving the signal from the fourth mixer, and commands the combiner to receive a demodulation signal output from the extra path thereby allowing the apparatus to simultaneously process the first and second operating frequency signals from the source and target bases stations, respectively.
In another aspect, when the apparatus moves from the operating area covered by both the source and target base stations to an operating area covered by the target base station, the controller causes the first local oscillator to adjust the first oscillation signal to demodulate the second operating frequency signal of the target base station and, upon stabilization of the adjusted first local oscillator, disables the extra path and commands the combiner to receive demodulation signals output from the plurality of main paths.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.